Fuel cells operating on the principal of the electrochemical reaction of hydrogen and a halogen material by means of catalytic electrodes are well known in the art. The fuel cells 1 are of the conventional design (see FIG. 1) comprising a membrane and electrode assembly 2 having a solid polymer electrolyte membrane 3 positioned between and in contact with a catalytic anode electrode 5 on one surface of the membrane 6 and a catalytic cathode electrode 7 positioned on the opposite surface of the membrane 8. Additionally, electrically conductive current collectors 9 and 10 are also present in the anode and the cathode chambers respectfully to facilitate the transfer of electrons to the cathode which have been produced at the anode.
In a single cell configuration end plates 11 and 12 are placed about and away from the anode 5 and the cathode 7 thereby forming a chamber 13 between the end plate and the anode and a chamber 14 between the cathode and the end plate. The anode chamber 13 receives the fuel through inlet 15 and the cathode chamber receives the catholyte through inlet 16 and the fuel can exit the cell through outlet 17 while the catholyte exits through outlet 18 during operation of the cell.
The operation of these fuel cells traditionally comprises introducing hydrogen gas into the anode chamber where it comes in contact with the catalytic anode and through the half reaction 1 produces hydrogen ions and electrons. ##STR1##
The electrons produced are gathered by the collector at the anode and transported to the cathode via an external circuit (or if a traditional bipolar collector is used the electrons are directed to a second cell cathode). The hydrogen ions produced at the anode are transported through the solid polymer electrolyte to the cathode side of the membrane.
At the cathode the catholyte is introduced into the cathode chamber. This catholyte contains the halogen material (bromine, chlorine or iodine) and is typically a solution of water and a halogenated compound such as hydrogen bromide, hydrogen chloride or hydrogen iodide. As the catholyte comes into contact with the catalytic cathode, it passes through the cathode chamber until it reaches the electrode/membrane/cathode interface 20 where it reacts with the hydrogen ions which have been transported through the membrane and the electrons from the anode in the presence of the catalyst thereby producing an acid. The cathodic reaction is shown in equation 2 below, for the halogen chlorine ##STR2##
One problem with these fuel cells is that the efficiency and performance deteriorates over time. It is thought that this is caused by the build-up of product acid at the cathode/electrolyte interface. It is felt that the acid concentration in these localized areas can become quite high. In the case of hydrochloric acid, it is thought that the concentration can exceed 7 molar HCl. Such a high concentration of acid at the interface may lead to a number of problems with the operation of the fuel cell such as increasing the ionic resistance of the membrane leading to voltage loss and accelerated voltage decay due to the local acid migrating to the anode and degrading the catalyst.
Ideally, the bulk catholyte introduced into the cathode chamber would be sufficient to dilute the acid produced at the interface. However, in order for the catholyte to dilute the acid it must first pass through the electrically conductive current collector/electrode support which is typically composed of porous plate of graphite/tantalum or multiple screens. These conductors provide a torturous path for the catholyte to pass through prior to reaching the cathode thereby severely limiting the flow of the bulk catholyte to the cathode and also limiting the ability of the catholyte to dilute the acid generated at the interface.
Therefore, what is required in this art is a method to reduce the build-up of the concentrated acid produced at the interface.